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Full text of "The manufacture and properties of iron and steel"

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236                         METALLURGY OF IROST AND STEEL.
.copper and nickel do not come Into this class, for their chemical similarity to iron prevents their separation.
Under ordinary circumstances the purification is so slight that it reduces the content of impurities in any part of the ingot hut little below the average, even though it may result in the serious contamination of the small region which is the last to solidify. This .arises from the fact that the surplus is concentrated in a very .small quantity of steel. Thus, if the ingot weighs 4000 pounds and contains 0,50 per cent, of carbon, the first 3900 pounds of steel which solidifies should contain 19.5 pounds of carbon, while the last 100 pounds should contain only 0.5 pound; but if there is a separation of two per cent, of the impurities during the chilling of the 3900 pounds, then this first portion will hold only 19.50.39= 19.11 pounds of carbon, a content of 0.49 per cent. The last 100 pounds will hold not only its fair proportion of 0.5 pound of carbon, but also the 0.39 pound rejected by the earlier solidifying part, and will therefore contain 0.89 per cent, of carbon. Thus a considerable degree of irregularity can be accounted for without assuming any attempt on the part of the metalloids to isolate themselves from the iron, but by supposing a regular separation of iron in obedience to the laws of crystallization.
In addition to this elimination of iron there is a definite process of separation and liquation on the part of the metalloids, which sometimes makes itself known in the formation of a very impure spot in the center of the mass, The exact circumstances under which this occurs to an excessive degree are not known. Slow cooling aids in the work, and the most marked cases are found in large masses of metal, but it is also true that both these conditions may exist without marked irregularity. The separation of the metalloids probably does not take place to any great extent until the external envelope of the ingot is of a considerable thickness, so that cooling is retarded. When it does occur, the compounds which are formed, being lighter than the mother metal, rise to the top, making the upper part of the ingot richer in metalloids than the normal. The lower part of the ingot will contain less than the average content of alloyed elements, since whatever excess is in the top must have been taken from the bottom.
For this reason the center of an ingot is not always homogeneous, but this irregularity is lessened in the subsequent working of the